Although not the same issue, a group it might be worth talking to for general advice on persuading the MCA to accept something unusual would be the Sea Change Sailing Trust based in Maldon. They are currently building (edit: in steel) a brand new Thames sailing barge to carry cargo under pure sail - no auxillary engine at all. I gather that, as you might imagine, this has involved some rather interesting discussions with the MCA.

I asked the Humber barges group on FB and none of the ex-bargemen there had heard of an ATIN... but there was a Sheffield size called NITA, owned by J J Tomlinsons.
So it looks like our intrepid photoshopper flipped a photo of NITA to get a more visually convincing result... and then flipped the resulting И to get ATIN!

Just to confirm your intuitions - that is definitely a Sheffield size motor keel, and the waterline on the bow corresponds to the 7ft draft mark on ours.
The next question though is whether the name has also been doctored. I have never heard of one called ATIN and can find no mention of it.

Indeed - also true of the Vetus ones posted above.
The Vetus one you posted first does need a hole. I'm familiar with them - I used one to replace a faulty mechanical one on the black tank of a tall ship I help out with. It was a drop-in replacement using the same SAE 5-hole mounting but still needs the cutout to look through. The BEP is similar.
The Tecma field effect sensors will work through a plastic tank but won't drive an actual gauge - they just tell you when the level has reached the height of the single sensor.
The Gobius unit does drive a gauge without needing a hole, but it's not an ultrasound unit - it's just just three field effect sensors used together. So it can distinguish between empty, above first/second/third sensor - driving a coarse LED gauge indicating in quarters - but won't give you any finer resolution than that as an ultrasound gauge would.
I stand by my advice that an ultrasound one, driving a gauge, as the OP asked about, requires a cutout.

They go on the outside of the tank, but you still need to drill a hole for them to look through and some screw holes for mounting. Same as you would for a mechanical float based one like the Wema/KUS units.
There is a BEP one which is slightly cheaper than the Vetus: http://www.cactusnav.com/ultrasonic-tank-sender-p-15649.html

If we were starting from scratch I might indeed do it differently. However with 20 MR11 fittings already neatly in place, I was hoping to just get some bulbs that worked rather than yet another project...

I am leaning in that direction, the problem is that it's impossible to get them open without breakage. The diffuser on the front is frosted glass, which has been epoxied in to the plastic housing.
I have destructively dismantled a dud one and extracted the board:
The design itself doesn't seem to be that bad. My guess is it has just been let down on lifetime, like so many products, by an overstressed electrolytic capacitor.
I have identified all the parts except the unmarked inductor - the driver is a PowTech PT4115 which should be more than adequate, with a Vishay MB6S rectifier and that 22uF cap in front of it to handle AC input (unnecessary in this installation). The diode in the step-down circuit is a Vishay SS14.
The bulb does run pretty warm. The ICs should be fine with that, but I would guess that the cap has failed from running at high temperature several hours a day for a year. It's labelled as rated to 105C, but as noted in this excellent app note from Maxim about caps in LED bulbs, that probably means it's only good to a couple thousand hours at that temperature, even assuming its ratings were verifiable in the first place. The capacitor is marked Jakec, who I don't know, but the first hit for "jakec capacitor" on Google is someone on badcaps.net saying they're crap...
So I would guess that I could fix these by just replacing the capacitors, if I could just get the casings apart without breaking them!

The second set of Chinese ones we tried were "COB", this style, but weren't that exact seller.
The problem with both those and the Osram ones is the narrow 35 degree beam. We've been much happier with the wide angle 120 degree ones which end up lighting the whole area rather than just the spot beneath them.

We have been struggling to find 12V MR11 LED bulbs that both suit our preferences and are reliable.
We've been through two sets I picked out from Chinese suppliers, the first set were a bit dim and too blue for us, the second set were brighter and warmer but too narrow-beamed and started failing after a few months.
We thought we'd finally hit the jackpot with these ones from UltraLEDs:
They're seriously bright (30 2835 LEDs, 350 lumen), warm colour temperature, and have a nice wide 120 degree spread with a diffuser on the front rather than just an exposed PCB with the LEDs on.
Having spent a bunch on getting 20 of them, I stuck a 12V regulator in the lighting circuit to protect them as it wasn't clear if they'd be OK with straight battery voltage.
All looked good but a year or so on more and more individual ones are starting to flicker - seems to be premature degradation of the internal drive circuitry, not the LEDs themselves. We're now losing more of them by the day and going to have to replace yet again.
Any ideas? These latest ones were perfect aside from the short lifetime. I have looked at Bedazzled etc but not turning up any as bright as this, their best MR11 is 18 2835 LEDs at 230 lumens.
I design electronics so would happily even make my own ones with decent drive circuits! But would need a source of MR11 housings and diffusers.

Would be good practice to put an elbow piece on that conduit where the flex goes up into the garage unit. You'd be surprised how easily water will dribble down the box, along the flex and all the way through the conduit to drip out of the other end.

Your diagram doesn't show any earth-neutral bond for the inverter.
Also do you really need separate 16A/32A hookup and a changeover switch? If you want to support both you could just have a 32A hookup, 32A cable, and an adapter you can plug in at the bollard end when you need to use a 16A supply.
That said, 32A supplies at visitor moorings are pretty rare so unless you have 32A at your home mooring, you're unlikely to ever need a 32A hookup.

Just to be clear on the actual limitations around London - at 2.75m your air draft would be about the same as ours. You would not be able to get up the Lea beyond the A104 bridge in Hackney, and that cuts off one of the main routes for London cruising. I don't know about the canals as we're too wide for those anyway, but a glance at Canalplan suggests you'd not even get to Uxbridge on the Grand Union - you'd be stuck at the bridge by Packet Boat marina.
Plenty of leisure cruising to be had for a big boat on the Thames, the Medway and around the estuary, but with a boat like that you can't do the "continuous cruising" thing round here.

Sorry, I think I have got this wrong. I thought the 2R and 3R were the 2:1 and 3:1 versions but I see now that all JP reduction boxes are 3R regardless of ratio. So I'm not sure if this one is a 3:1 or 2:1 now. Unless someone can tell from the photos, I'll turn it over to check.

Hi Admiral,
We might be able to do the swap you're looking for. We are currently stripping down a 1952 JP3M that came with a 3R box on it, but the one in our boat is a 2R, so a spare for that would suit us better.
See photos below. The engine was supposedly taken out of a barge in running condition a few years ago. We haven't found anything that suggests otherwise, but we bought it as-is and are stripping down to check everything properly. We haven't had the box off the engine yet but that's probably one of the next jobs.
We're in east London. PM me if you're interested.